1. Field of the Invention
The present invention relates to a battery charger for charging batteries that are used as the power source of a portable device, such as a cordless power tool. Examples of batteries include nickel cadmium batteries, and nickel hydrogen batteries
2. Description of the Related Art
Recently, nickel cadmium batteries, nickel hydrogen batteries, and other secondary batteries are being produced with a large capacity and improved charge characteristic when charged with a large current.
The cycle life of a battery can be reduced if the battery is allowed to overheat during charge. To prevent this potential reduction in cycle life, battery chargers stop charging operations, once the subject battery has heated up to a predetermined temperature, of 60.degree. C. for example. However, large capacity batteries generate more heat when charged than do batteries with a smaller rated capacity. When large rated capacity batteries are charged using the same large charge current as used for charging small rated capacity batteries, then large rated capacity batteries heat up and exceed the predetermined temperature before being fully charged. Because the charging operations are stopped at this point, the battery is less than fully charged.
For this reason, a battery charger has been proposed that charges small rated capacity batteries using a relatively large charge current and charges large rated capacity batteries using a relatively small charge current. However, such a battery charger takes a long time to charge large rated capacity batteries.
Another charger is provided with cooling device, such as a cooling fan, for cooling down the battery while it is being charge. With this configuration, even large rated capacity batteries can be quickly charged using a relatively large charge current without damaging the battery. However, this type of battery charger is, as a matter of course, more expensive than battery chargers without cooling device, and so is unappealing to users that only rarely need to charge large rated capacity batteries. Further, battery chargers without cooling device are quite capable of properly charging large rated capacity batteries, although they require longer to do so. Therefore, there is no absolute necessity for battery chargers to have cooling device. For such reasons, both battery chargers with cooling device and battery chargers without cooling device are present on the market. This requires a manufacturer or retailer to provide various support, such as repair and maintenance support, for both battery chargers with cooling device and battery chargers without cooling device. Also, producing both types is not very economical, and productivity is poor.